The invention relates to a method for estimating the quality of a connection in a digital radio system in which the signals transmitted comprise bursts consisting of symbols and comprising a known training sequence, and in which method a received signal is processed by a matched filter.
In a cellular radio system, the quality of a connection between a base station and a subscriber terminal varies continuously. The variation is caused by interference on the radio path and attenuation of radio waves as a function of distance and time on a fading channel. The quality of a connection can be measured, for example, by monitoring the power received. Any variation in the connection quality can be partly compensated for by power adjustment.
In a digital cellular radio system, a more accurate method than power measurement is needed in order to estimate the quality of the connection. Previously known quality parameters are here, for example, Bit Error Rate BER and signal-to-noise ratio.
It is previously known to utilize Viterbi detection decisions in the estimation of a signal-to-noise ratio of a received signal. A base station or a subscriber terminal can function as a receiver. In previously known solutions, Viterbi detection is performed on a received burst as a whole before the signal-to-noise ratio is defined. Since a Viterbi algorithm, however, is often too demanding an operation for a digital signal processing program to perform in the processing time allowed by the receiver, separate Viterbi hardware has to be used for it. This is described in greater detail in xe2x80x98A Viterbi Algorithm with Soft-decision Outputs and its Applicationsxe2x80x99 by J. Hagenauer and P. Hoeher in IEEE GLOBECOM 1989, Dallas, Tex. November 1989, which is incorporated herein by reference.
In cellular radio systems, various diversity methods are used generally for improving the quality of a connection between a base station and a subscriber terminal under varying conditions. In diversity methods, information contained in a plural number of received signals that do not correlate with one another is preferably combined. In multipath reception, the most common diversity receivers combine the signals before or after the detection, and they comprise, for example, selective combination, maximal ratio combination, and equal gain combination. Multipath signals are usually detected by Viterbi detection, whereby they are combined after the detection. The diversity receivers and the above methods are described in greater detail in, for example, William C. Y. Lee, Mobile Communications Engineering, chapter 10, xe2x80x98Combining technologyxe2x80x99, p. 291-336, Mc-Graw-Hill, U.S.A., 1982, which is incorporated herein by reference.
The preferred diversity combination method is maximal ratio combination, which is conducted before the detection, since a higher signal gain is then achieved. In the maximal ratio combination, the received signals are weighted and co-phased before the combination. The problem lies in finding the correct weighting coefficients.
The object of the present invention is to implement a method by which it is possible to estimate the quality of a connection by a quantity proportional to the signal-to-noise ratio directly from the received signal without the use of Viterbi detection. Another object is to implement a method for computing weighting coefficients and thereby to enable signal combination before detection, when diversity receivers are used.
The above is achieved by a method according to the introduction, which is characterized in that a result vector of at most the length of the training sequence is formed from the symbols of the output signal of the matched filter and the corresponding symbols of the known training sequence, and that a mean value and variance are computed for the symbols of the result vector, and that a quantity proportional to signal quality is formed as a quotient of the squared mean value of the result vector and the variance of the result vector.
The invention also relates to a diversity combination method according to claim 2 and a receiver according to claim 5.
Several advantages are achieved by the solution provided by the invention. By the method of the invention, the signal quality can be estimated directly from the received signal without conducting Viterbi detection, or by carrying out the method of the invention before Viterbi detection, utilizing the previously known training sequence contained in the burst. The invention comprises two main steps: forming of a result vector by a matched filter and a training sequence, and forming of a quantity proportional to signal quality by the result vector. The quantity proportional to signal quality stands for the signal-to-noise ratio.
When the use of a Viterbi algorithm is avoided, memory capacity is saved and less time is needed for computation. The quantity proportional to signal quality can be used in the estimation of the state of the channel, in methods for estimating bad frames, and particularly in the computation of weighting coefficients in diversity combination when multipath signals are combined before detection.